Coated granular magnetite particles and process for producing the same

Abstract: Coated granular magnetite particles each comprise a granular magnetite core particle coated with a layer of M x Fe 2+y O z wherein M is Zn or Co, 0.4 ≦ x ≦ 1, x + y = 1, 4.0 ≦ z ≦ 4.3 and the amount of M in said M x Fe 2+y O z is from 0.5 to 6 mol% based on the Fe content of the core particle.

Description: The present invention relates to coated granular magnetite particles and a process for producing the same. Coated granular magnetite particles according to the present invention are chiefly intended as materials for coloring pigments for coating materials, printing inks and resins, and materials for magnetic toners and magnetic carriers. Since granular magnetite particles are black, they are widely used as materials for a black coloring pigment which is dispersed in a vehicle or mixed with a resin. In addition, since granular magnetite particles are black ferrimagnetic particles, they are mixed with and dispersed in a resin so as to produce composite particles which are also widely used as a particle material for magnetic toners and magnetic carriers for electrostatic copying. When granular magnetite particles are used as a material for a black coloring pigment, if the granular magnetite particles have too high a coercive force, magnetic reagglomeration is produced among the particles, which makes the dispersion of the particles in a vehicle or a resin difficult. From the point of view of dispersibility, granular magnetite particles are required to have a coercive force of not more than about 300 Oe. When granular magnetite particles are used as a material for a magnetic toner, (1) it is preferable that the granular magnetite particles have a high coercive force in order to improve the carrying property and the fluidity of the magnetic toner, but in order to improve the dispersibility of the magnetic particles and the turbulence of the magnetic toner, to activate the contact, to improve the separation and the friction both between the magnetic particles and between the magnetic particles and the sleeve of the developing equipment, and to improve the developing property, the coercive force thereof must be not more than about 300 Oe. (2) On the other hand, when used as a material for a magnetic toner, in order to prevent the reduction in the image density in continuous copying and to prevent defective development such as a part of the image being missing, the coercive force of the granular magnetite particles should preferably be as low as possible, particularly not more than 150 Oe. As described above, granular magnetite particles are widely used as materials for a black coloring pigment. Since they are often exposed to a high temperature of not lower than 150°C, particularly not lower than 200°C, they are required to keep a stable tone at a temperature of not lower than 150°C, particularly not lower than 200°C (this property is referred to as "heat resistance" hereinunder). This fact is described in Japanese Patent Publication (KOKOKU) No. 54-7292 (1979) as " ··· When the pigment is used as a coloring agent for a thermoplastic resin such as polyethylene, polypropylene, polystyrene and ABS, since these thermoplastic resins are formed and processed at a temperature as high as approximately more than 200°C, especially a yellow iron oxide pigment is not immune to discoloring, so that the uses thereof are limited. ···" and in JP-A- 55-65406 (1980) as "Such magnetic powder for a one component magnetic toner is generally required to have the following properties. ··· iv) To have a blackness which stands practical use. Although it is possible to add a coloring agent to the magnetic toner, it is preferable that the magnetic powder itself has a black color and that no coloring agent is used. v) To have a high heat resistance. It is necessary that the tone, particularly the blackness and the electromagnetic characteristics are sufficiently stable in a temperature range of 0 to about 150°C. ···" The strength of the black colour of granular magnetite particles depends upon the Fe 2+ content and is apt to become stronger with the increase in the Fe 2+ content. The cause of the tone of the granular magnetite particles changing from black to brown is the oxidation of Fe 2+ in the granular magnetite particles to Fe 3+ , thereby transforming the granular magnetite particles into maghemite particles when the granular magnetite particles are exposed to a temperature as high as 150°C, especially to a temperature of about 200°C. Furthermore, since if it is possible to achieve the desired color with a small amount of granular magnetite particles, it is advantageous because it saves resources and energy as well as improving the handling properties. Granular magnetite particles are required to maintain the strengh of black colour throughout heating as much as possible so as to have a high tinting strength. Granular magnetite particles are conventionally produced by what is called a wet-process which comprises the steps of: blowing an oxygen-containing gas into a suspension containing Fe(OH) 2 colloids or Fe-containing precipitates which are obtained by adding an aqueous ferrous salt solution to an aqueous alkali solution such as an aqueous alkali hydroxide solution and an aqueous alkali carbonate solution. Various attempts have been made to improve the properties of granular magnetite particles. For example, methods are known of coating granular magnetite particles with an insoluble inorganic compound such as zinc hydroxide, zinc phosphate, zinc phosphite, aluminum phosphate and silica (US-A-4,082,905), a method of coating granular magnetite particles with a ferrite containing 1.5 to 13 mol% of a divalent metal other than Fe JP-A-3-67265 (1991)), and a method of coprecipitating an oxide of Zn, Mn, Ni, Co, Mg, Cu or Cd when producing granular magnetite particles (Japanese Patent Publication (KOKOKU) No. 3-48505 (1991)). More specifically, US-A-4,082,905 discloses an Fe 3 O 4 iron oxide black pigment having incorporated therein up to about 50 mole percent of γ-Fe 2 O 3 plus MeFe 2 O 4 (wherein Me is one of magnesium, manganese, cobalt, nickel, aluminum, chromium, titanium, copper, zinc and cadmium) having a specific surface according to BET of greater than about 12 m 2 /g, up to about 10% by weight of a substantially insoluble inorganic compound as coating and a stability to oxidation with air under standardized conditions of greater than about 130°C. JP-A-3-67265 (1991) discloses a magnetic toner for developing an electrostatic image, comprising at least a binder resin and a spherical magnetic powder containing spherical magnetic particles, which have a surface layer composed of a different composition from the core portion, the surface layer being composed of a ferrite containing 1.5 to 13 mol% (calculated as divalent metal ions) of a divalent metal oxide other than iron. Japanese Patent Publication No. 3-48505 (1991) discloses a magnetic toner for developing an electrostatic latent image, comprising a binder resin and a magnetic powder as the main ingredients, the magnetic powder being produced by a wet-process and containing an oxide of Zn, Mn, Ni, Co, Mg, Cu or Cd coprecipitated in the process of production. As one method of improving the properties of the magnetic particles, "an acicular ferromagnetic iron oxide particles for a magnetic recording medium, comprising Co-coated γ-Fe 2 O 3 particles as a core and an iron oxide layer containing zinc and a ferrous formed on the surfaces of Co-coated γ-Fe 2 O 3 particles" is known in the field of a magnetic particles for magnetic recording, as disclosed in JP-A-60-165703 (1985). However, a method of coating magnetite particles with zinc and/or cobalt ferrite has not yet been proposed in the field of pigments. Granular magnetite particles having as low a coercive force as possible, particularly a coercive force of not more than 300 Oe and yet having high heat resistance and tinting strength are in strong demand at present, but the granular magnetite particles obtained in any of the above-described methods have poor heat resistance and tinting strength, although they have a coercive force which is as low as 40 to 130 Oe. These granular magnetite particles begin to transform into maghemite particles at a temperature of about 130°C, and the color changes from black to liver brown, so that the tinting strength is low, as shown in the comparative examples which will be described later. Although the particles described in US-A-4,082,905 have an improved heat resistance, it cannot be said to be satisfactory. In the particles described in JP-A-3-67265 (1991), the amount of charge can be controlled, but the heat resistance is not improved, and the particles begin to transform into maghemite particles at a temperature of about 140°C, as shown in Comparative Example 4 which will be described later. That is, the particles have a poor heat resistance. The granular magnetite particles described in Japanese Patent Publication Nos. 59-43408 (1984) and 3-48505 (1991) are not the particles with improved heat resistance and tinting strength which comprise the present invention. The above-described coating of the acicular magnetic particles with zinc ferrite JP-A-60-165703 (1985)) in the field of magnetic recording improves the saturation magnetization and the change of the saturation magnetization with time in response to the demand for high-density recording, but the improvement of heat resistance and tinting strength which is achieved by the present invention, is not the aim at all. Actually, these particles have a low heat resistance and begin to transform into maghemite particles at a temperature of about 120 to 145°C, as shown in the comparative examples which will be described later. As to the charging property of the particles, it is preferable that the magnetic particles are positively charged in the case of a positive charge toner or that the magnetic particles are negatively charged in the case of a negative charge toner, as described in JP-A-60-117259 (1985) as "On the other hand, it is necessary that the toner is adjusted so as to be positively or negatively charged depending on the kind of a photosensitive material. Magnetic particles such as magnetite particles have a property of negatively charging. When such magnetic particles are used for a magnetic toner provided with a positive charging property, the phenomenon that a positively charged portion and a negatively charged portion coexist on the surface of one toner particle or the charging properties are different in toner particles, may be produced due to the existence of the magnetic particles exposed on the surface of the toner. This phenomenon is supposed to be one cause of abnormality in the image copying properties under some circumstance in a one component developing system using a magnetic toner. It is, therefore, expected that the magnetic particles in the toner are positive charge particles." It is also preferred that the amount of charge of the toner is adjusted in an appropriate range so as to fit a developing equipment. For this purpose, the amount of charge of the whole magnetic particles is adjusted to a adequate amount. As a result of various studies undertaken by the present inventors so as to solve the above-described problems in the related art, granular magnetite particles have been obtained coated with M x Fe 2+y O z (wherein M is Zn or Co, 0.4 ≤ x≤ 1, x + y = 1, and 4.0 ≤ z ≤ 4.3), which have a change ratio (%) in the Fe 2+ content of not more than 8% and a change in hue of not more than 0.8 after heating the coated granular magnetite particles at a temperature of 200°C for 1 hour, and are black and have a very high heat resistance. The present invention has been achieved on the basis of this finding. It is an object of the present invention to provide coated granular magnetite particles which have a low coercive force, especially a coercive force of not more than 300 Oe, an excellent heat resistance and a high tinting strength. It is another object of the present invention to provide coated granular magnetite particles in which the change ratio (%) in the Fe 2+ content is of not more than 8% and the change in hue is of not more than 0.8 after heating the coated granular magnetite particles at a temperature of 200°C for 1 hour, and which have a high black coloring power. It is still another object of the present invention to provide coated granular magnetite particles which have a preferred amount of charge. To achieve these aims, in a first aspect of the present invention, there are provided coated granular magnetite particles each comprising a granular magnetite core particle, coated with a layer of M x Fe 2+y O z wherein M is Zn or Co, 0.4 ≤ x ≤ 1, x + y = 1, 4.0 ≤ z ≤ 4.3 and the amount of M in said M x Fe 2+y O z is from 0.5 to 6 mol% based on the Fe content of the core particle. In a second aspect of the present invention, there are provided coated granular magnetite particles as in the first aspect wherein M is Zn and the amount of Zn in the Zn x Fe 2+y O z is from 0.5 to 4.0 mol% more preferably from 1 to 4 mol%, based on the Fe content of the core particle. In a third aspect of the present invention, there are provided coated granular magnetite particles as in the first aspect wherein M is Co and the amount of Co in the CO x Fe 2+y O z is from 1.0 to 6.0 mol%, more preferably from 2 to 6 mol%, based on the Fe content of the core particle. In a fourth aspect of the present invention, there are provided coated granular magnetite particles comprising: granular magnetite particle as a core; Zn x Fe 2+y O z as a surface layer, wherein 0.4 ≤ x ≤ 1, x + y = 1, and 4.0 ≤ z ≤ 4.3, the amount of Zn in said Zn x Fe 2+y O z being 0.5 to 4.0, more preferably from 1 to 4 mol%, based on the total Fe content in said granular magnetite particle as a core; and a coating layer formed on said Zn x Fe 2+y O z surface layer, and composed of a coprecipitate of a hydroxide, a hydroxide oxide or a mixture thereof of at least one element selected from the group consisting of Si, Al and Ti, the amount of said coprecipitate being 0.1 to 5 wt% (calculated as Si, Al and/or Ti) based on said coated granular magnetite particle. In a fifth aspect of the present invention, there are provided coated granular magnetite particles comprising: granular magnetite particle as a core; Co x Fe 2+y O z as a surface layer, wherein 0.4 ≤ x ≤ 1, x + y = 1, and 4.0 ≤ z ≤ 4.3, the amount of Co in said Co x Fe 2+y O z being 1.0 to 6 mol%, more peferably from 2 to 6 mol%, based on the total Fe content in said granular magnetite particle as a core; and a coating layer formed on said Zn x Fe 2+y O z surface layer, and composed of a coprecipitate of a hydroxide, a hydroxide oxide or a mixture thereof of at least one element selected from the group consisting of Si, Al and Ti, the amount of said coprecipitate being 0.1 to 5 wt% (calculated as Si, Al and/or Ti) based on said coated granular magnetite particle. In a sixth aspect of the present invention, there is provided a process for producing coated granular magnetite particles comprising the steps of: (a) forming an aqueous dispersion of granular magnetite core particles ; (b) adding to and mixing with the said dispersion in a non-oxidising atmosphere from 1.0 to 39 mol% of an aqueous ferrous salt solution (calculated as Fe 2+ and based on the total amount of Fe of the core particle), 0.5 to 6 mol% of an aqueous M salt solution (calculated as M and based on the amount of Fe of the core particle) and an aqueous alkali hydroxide solution, thereby adjusting the concentration of OH - groups in said aqueous dispersion to from 0.3 to 1.0 mol/l; (c) blowing a molecular oxygen-containing gas into said dispersion at a temperature of not lower than 50°C to coat the surfaces of said core particles to be treated with M x Fe 2+y O z , wherein M is Zn or Co, 0.4 ≤ x ≤ 1, x + y = 1, and 4.0 ≤ z ≤ 4.3; and optionally carrying out (d) and (e) which comprise (d) adding at least one compound selected from the group consisting of an Si compound, an Al compound and a Ti compound to the reaction system from step (c) ; and (e) adjusting the pH of the resultant aqueous dispersion to from 5 to 9 to coat each of said M x Fe 2+y O z layers with a coprecipitate of a hydroxide, a hydroxide oxide or a mixture thereof of at least one element selected from the group consisting of Si, Al and Ti. The granular magnetite particles to be treated (to be coated) in the present invention are black particles containing 12 to 24 wt% of Fe 2+ , which are obtained by a wet-process. The particle shape of the granular magnetite particles to be treated is not restricted, and the particles may be, for example, spherical, cubic or octahedral. The particle size of the granular magnetite particles to be treated is preferably in the range of 0.1 to 0.5 µm and the BET specific surface area of the core particle is from 3 to 15 m 2 /g with consideration for the dispersibility of the particles. The granular magnetite particles to be treated containing 12 to 24 wt% of Fe 2+ are ordinarily black, but in order to produce the desired coated particles which have a stronges black color and a higher tinting strength, it is preferable to use granular magnetite particles containing a larger amount of Fe 2+ , for example particles wherein the Fe 2+ content in the core particle is from 14 to 24 wt%, more preferably 17% to 24%. The surface layer is composed of M x Fe 2+y O z (wherein M is Zn or Co, 0.4 ≤ x ≤ 1, x + y = 1, and 4 ≤ z ≤ 4.3). If x is less than 0.4, the heat-resistance obtained is insufficient, and the exothermic reaction initiating-temperature is lowered. On the other hand, if x exceeds 1.0, the M ions which do not contribute to the production of the coating material unfavorably separate out in the form of fine M(OH) 2 particles. The amount of M in M x Fe 2+y O z is 0.5 to 6 mol% based on the total Fe content in the granular magnetite particles to be treated. If the amount of M is less than 0.5 mol%, the improvement of heat resistance is insufficient, and the exothermic reaction initiating-temperature is lowered. If it exceeds 6 mol%, the heat resistance is improved, but the coercive force sometimes unfavorably exceeds 300 Oe. There is a tendency of the heat resistance of the coated granular magnetite particles improving and the coercive force thereof increasing as the amount of M in M x Fe 2+y O z increases and the value of x approaches 1. The Fe content in M x Fe 2+y O z may be an amount which is necessary for producing M x Fe 2+y O z , preferably 1 to 39 mol% based on the total Fe content in the granular magnetite particles to be treated. In the process of the invention, the addition of the aqueous ferrous salt solution, the aqueous M salt solution and the aqueous alkali hydroxide solution can be in any order. When M is Co, the amount of aqueous cobalt salt solution is preferably 1 to 6 mol%, more preferably 2 to 6 mol% (calculated as Co) based on the total amount of Fe in the granular magnetite particles to be treated. The amount of aqueous ferrous salt solution may be an amount which is necessary for producing Co x Fe 2+y O z . It is preferably 2 to 39 mol%, more preferably 4 to 39 mol% (calculated as Fe 2+ ) based on the total Fe content in the granular magnetite particles to be treated. When M is Zn, the amount of aqueous zinc salt is preferably 0.5 to 4.0 mol%, more preferably 1 to 4 mol% (calculated as Zn) based on the total amount of Fe in the granular magnetite particles to be treated. The amount of aqueous ferrous salt may be the minimum which is necessary for producing Zn x Fe 2+y O z . It is preferably 1.0 to 26 mol%, more preferably 2.0 to 26 mol% (calculated as Fe 2+ ) based on the total Fe content in the granular magnetite particles to be treated. The aqueous ferrous salt solution, aqueous M salt solution and aqueous alkali hydroxide solution are added to and mixed with the aqueous dispersion containing the granular magnetite particles to be treated, in a non-oxidizing atmosphere. In order to produce a non-oxidizing atmosphere, nitrogen gas or the like is blown into the reaction vessel. If the atmosphere is not a non-oxidizing atmosphere, M x Fe 2+y O z does not sufficiently grow on the surfaces of the granular magnetite particles and fine M x Fe 2+y O z particles separate out. Ferrous sulfate, ferrous chloride, can be used as the aqueous ferrous salt solution. As the aqueous cobalt salt solution used in the present invention, cobalt sulfate, cobalt chloride, cobalt hydroxide, etc. are usable. As the aqueous zinc salt solution used in the present invention, zinc sulfate, zinc chloride, zinc nitrate, zinc phosphate etc. are usable. As the alkali hydroxide solution used in the present invention, sodium hydroxide, potassium hydroxide, etc. are usable. The amount of alkali hydroxide solution is adjusted so that the concentration of the OH group in the dispersion is 0.3 to 1.0 mol/l, preferably 0.5 to 1.0 mol/l. If the concentration of the OH group is less than 0.3 mol/l, M x Fe 2+y O z does not sufficiently grow on the surfaces of the granular magnetite particles and fine M x Fe 2+y O z particles separate out. In order to grow M x Fe 2+y O z on the surfaces of the granular magnetite particles 1.0 mol/l of alkali hydroxide solution is sufficient as the concentration of the OH group in the dispersion. As the oxygen-containing gas used in the present invention, air is optimal. The oxidation temperature in the present invention is now lower than 50°C, preferably not lower than 70°C, more preferably 70 to 100°C. If the temperature is lower than 50°C, not all of the added ferrous salt contributes to the production of the zinc ferrite coating layer and/or cobalt ferrite coating layer, and acicular goethite particles or acicular lepidcrocite particles are produced from a portion of the added ferrous salt and contaminate the coated granular magnetite particles. Thus obtained coated granular magnetite particles according to the present invention have an average particle diameter of 0.1 to 0.5 µm. The coated granular magnetite particles of the present invention include the coated granular magnetite particles comprising a further coating layer which is present on the M x Fe 2+y O z layer, and which comprises a hydroxide, a hydroxide oxide or a mixture thereof of at least one element selected from Si, Al and Ti in an amount from 0.1 to 5 wt% (calculated as Si, Al and/or Ti) based on the coated granular magnetite particles. The amount of coprecipitate of the hydroxide and/or hydroxide oxide thereof is preferably 0.1 to 4 wt% (calculated as Si, Al and/or Ti) based on the coated granular magnetite particles. The coprecipitate layers are formed on the surface of the M x Fe 2+y O z layer by adding an Si compound, an Al compound and/or Ti compound to the reaction system after the M x Fe 2+y O z layers are formed, in other words, after the surfaces of the granular magnetite particles are coated with M x Fe 2+y O z by blowing an oxygen-containing gas into the dispersion at a temperature of not lower than 50°C; and adjusting the pH of the resultant aqueous dispersion to 5 to 9, preferably 6 to 8. Examples of the Si compound used in the present invention are water glass #3, sodium silicate and potassium silicate. Examples of the Al compound used in the present invention are aluminum sulfate, aluminum chloride, aluminum nitrate and sodium aluminate. Examples of the Ti compound used in the present invention are titanyl sulfate and titanium chloride. The amount of Si compound, Al compound and/or Ti compound added to the reaction system is 0.1 to 5.0 wt%, preferably 0.1 to 4 wt% (calculated as Si, Al and/or Ti) based on the coated granular magnetite particles. In order to adjust the pH of the aqueous dispersion containing the Si compound, the Al compound and/or the Ti compound to 5 to 9, an acid (such as sulfuric acid, hydrochloric acid, nitric acid, etc.) or a base (such as sodium hydroxide, potassium hydroxide, etc.) is added as needed. Therefore an important point in the present invention is the fact that when granular magnetic particles which are to be treated are dispersed in an aqueous dispersion; 1.0 to 39 mol% (calculated as Fe 2+ and based on the total amount of Fe in the granular magnetite particles to be treated in the aqueous dispersion) of an aqueous ferrous salt solution, 0.5 to 6 mol% (calculated as M and based on the total amount of Fe in the granular magnetite particles to be treated) of an aqueous M salt solution and an aqueous alkali hydroxide solution are added to and mixed with said aqueous dispersion in a non-oxidizing atmosphere to adjust the concentration of the OH group in said aqueous dispersion to 0.3 to 1.0 mol/l, or 1.0 to 39 mol% (calculated as Fe 2+ and based on the total amount of Fe in the granular magnetite particles to be treated in the aqueous dispersion) of an aqueous dispersion and 0.6 to 6.0 mol% (calculated as M and based on the total amount of Fe in the granular magnetite particles to be treated) of an aqueous M salt solution are added to and mixed with the obtained aqueous dispersion in a non-oxidizing atmosphere, and the concentration of the OH group in the resultant dispersion is adjusted to 0.3 to 1.0 mol/l, preferably 0.5 to 1.0 mol/l by adding an aqueous alkali hydroxide solution in a non-oxidizing atmosphere; and an oxygen-containing gas is blown into the dispersion at a temperature of not lower than 50°C, it is possible to obtain granular magnetite particles coated with M x Fe 2+y O z (wherein M is Zn or Co, 0.4 ≤ x≤ 1, x + y = 1, and 4.0 ≤ z ≤ 4.3), the amount of M being 0.5 to 6 mol% based on the total Fe content in the granular magnetite particles to be treated, and the coated granular magnetite particles obtained having a coercive force of not more than 300 Oe and excellent heat resistance and tinting strength. The mechanism of improving the above-described properties of the coated granular magnetite particles of the present invention has not been theoretically elucidated. However, since coated granular magnetite particles with the desired properties have not been obtained when the atmosphere in which an aqueous ferrous salt, an aqueous M salt and an aqueous alkali hydroxide are added to and mixed with the obtained aqueous dispersion containing the granular magnetite particles to be treated is an oxidizing atmosphere, or when the concentration of the OH group is out of the range of 0.3 to 1.0 mol/l, as shown in comparative examples which will be described later, it is considered that a uniform and dense M x Fe 2+y O z layer is formed on the surface of the granular magnetite particle to be treated due to the synergistic effect of the non-oxidizing atmosphere which controls the oxidation reaction so as to enable a uniform oxidation in the reaction system and the adjustment of the concentration of the OH group to a specific range to lower the solubility of the hydroxide, thereby preventing the separation of a new nucleus crystal, and that the uniform and dense M x Fe 2+y O z layers contribute to the improved properties of the granular magnetite particles. The exothermic reaction initiating-temperature of the coated granular magnetite particles of the present invention was not lower than 150°C, preferably not lower than 200°C, as is shown in examples which will be described later. The change ratio (%) in the Fe 2+ content was not more than 8%, preferably not more than 6.5%, more preferably not more than 6.0% after heating the granular magnetite particles even at a temperature as high as 200°C for 1 hour. That is, the Fe 2+ content was similar before and after heating. As a result, ΔE (change of hue) was not more than 0.8, preferably not more than 0.65, more preferably not more than 0.60. The strength of black colour was also sufficiently maintained and the heat resistance was excellent. The BET specific surface area of the coated granular magnetite particles of the present invention is not more than 15 m 2 /g, preferably 3 to 15 m 2 /g; the coercive force is generally not more than 300 Oe, especially, when M is Co, the coercive force is preferably 150 to 300 Oe, more preferably 170 to 300 Oe, and when M is Zn, the coercive force is preferably not more than 150 Oe, more preferably 30 to 150 Oe; the residual magnetization and the saturation magnetization are not more than 28 emu/g and not less than 80 emu/g, respectively; when M is Co, the residual magnetization is 15 to 28 emu/g, more preferably 18 to 28 emu/g, and when M is Zn, the residual magnetization is not more than 15 emu/g, more preferably 2 to 15 emu/g. If the coated granular magnetite particles with the M x Fe 2+y O z layers coated with coprecipitates of a hydroxide and/or hydroxide oxide of at least one element selected from the group consisting of Si, Al and Ti are used for a magnetic toner, the surface of the magnetic toner is uniformly charged, the particles are capable of being charged to from -50 to 15 µC/g, and the heat resistance is further improved. The particles of the invention can have a coercive force of not more than 300 Oe, a change after heating at a temperature of 200°C for 1 hour of the Fe 2+ content of not more than 6.5% and a hue of not more than 0.65. The coated granular magnetite particles according to the present invention have an excellent heat resistance and a high tinting strength, so that coating materials, printing inks and resins can incorporate the particles as a colouring pigment, and the particles can be incorporated in magnetic toners and magnetic carriers. Examples The present invention will be explained in more detail hereinunder with reference to the following examples and comparative examples. The average particle diameter in the examples and comparative examples were measured by a BET method. The Co content and the Fe content in the particles was measured by a high-frequency plasma emission spectrophotometer ICAP-575 (manufactured by Nihon Jarrel Ash Co., Ltd). The magnetic characteristics of the particles were measured under an external magnetic field of 10 KOe by a vibration sample magnetometer VSM-3S-15 (manufactured by Toei Kogyo Co., Ltd). The heat resistance of the particles was expressed as the exothermic reaction initiating-temperature (°C) measured by differential thermal analysis using a differential scanning calorimeter DSC-200 (manufactured by Seiko Instruments and Electronics, Ltd.). The change ratio (%) in the Fe 2+ content after heating the granular magnetite particles at a temperature of 200°C for 1 hour was measured. The L 1 * value, the a 1 * value, and the b 1 * value before heating and the L 2 * value, the a 2 * value, and the b 2 * value after heating were measured so as to express the change in the hue by the following formula: ΔE = (L 2 * - L 1 *) 2 + (a 2 * - a 1 *) 2 + (b 2 * - b 1 *) 2 The hue was expressed by the L* value (lightness), the a* value and the b* value. These values were measured in Hunter's Lab space by using a light-source spectrophotometric colorimeter MSC-1S-2D (manufactured by Suga Testing Machine, Ltd.), and expressed in accordance with the uniform perceived color space determined by Commission Internationale de l'Exlairage, 1976. The test specimen for measuring the hue was obtained by kneading 0.5 g of the coated granular magnetite particles and 0.5 cc of castor oil in the form of a paste using a Hoover muller, adding a clear lacquer to the paste and kneading the mixture so as to produce a coating material, and applying the coating material onto a cast-coated paper by using a 6-mil applicator. Example 1 0.24 mol of a ZnSO 4 solution, 0.50 mol of an FeSO 4 solution and 37.0 mol of an aqueous NaOH solution were added to a dispersion of 90°C which was obtained by dispersing 1856 g of octahedral magnetite particles (average particle diameter: 0.26 µm, coercive force: 120 Oe, BET specific surface area: 5.5 m 2 /g, Fe 2+ content: 20.1 wt%) produced from an aqueous solution (by wet-process) into water, while blowing N 2 gas into the dispersion at a rate of 20 liter/min, so that the total content was 39.5 liter. The Zn 2+ content and the Fe 2+ content in the obtained dispersion were 1.00 mol% and 2.09 mol%, respectively, based on the total Fe content in the granular magnetite particles, and the concentration of the OH group was 0.94 mol/liter. Thereafter, air was blown into the resultant dispersion in place of the N 2 gas at a rate of 100 liter/min, while the resultant dispersion was stirred at a temperature of 90°C for 20 minutes, thereby coating the granular magnetite particles with zinc ferrite. The black precipitates obtained were filtered out, washed with water and dried at a temperature of 60°C to obtain black particles. As a result of the measurement and the composition analysis of Zn and Fe by the method which will be described in the following, the composition of the surfaces of the obtained black particles was represented by the general formula Zn x Fe 2+y O z , wherein X = 0.97 and 2 + y = 2.03, and the Zn content in the Zn x Fe 2+y O z layer was 1.0 mol% based on the total Fe content in the black particles to be treated and the Fe content in the coating material was 2.1 mol% based on the total Fe content in the black particles to be treated. The BET specific surface area of the obtained black particles was 4.9 m 2 /g and the Fe 2+ content thereof was 20.5 wt%. As to the magnetic characteristics of the obtained black particles, the coercive force (Hc) was 124 Oe, the residual magnetization (σ r ) was 12.8 emu/g, and the saturation magnetization (σ s ) was 85.4 emu/g. The exothermic reaction initiating-temperature measured from differential thermal analysis was 219°C. In other words, the coated granular magnetite particles had excellent in heat resistance. The Fe 2+ content after heating the granular magnetite particles at a temperature of 200°C for 60 minutes was 19.6 wt%. Since the Fe 2+ content before heating was 20.5 wt%, the change ratio (%) in the Fe 2+ content was as small as 4.4 %. The granular magnetite particles assumed a bluish black color, and the tinting strength was excellent. ΔE was 0.42. For the composition analysis, 10 g of the obtained black particles were suspended in 100 ml of water, and the resultant suspension was heated to 60°C in a reaction vessel. Thereafter, 200 ml of 1-N HCl solution was added to the suspension under stirring so as to measure the Zn content and the Fe content. Six test samples were prepared and were taken out one after another t = 0, 1, 5, 10, 30, 60, and 120 minutes after the addition of the HCl solution to analyze the Zn content and the Fe content in the filtrate obtained by filtering out the black particles. Examples 2 to 10, Comparative Examples 1 to 4 The coated granular magnetite particles were obtained in the same manner as in Example 1 except for varying the kind of the granular magnetite particles, the kinds and the amounts of aqueous zinc salt solution, aqueous ferrous salt solution and aqueous NaOH solution in the mixing process, and the temperature in the oxidation process. The synthesis conditions and the resultant properties of the coated granular magnetite particles obtained are shown in Tables 1 to 3. Example 11 0.84 mol of a CoSO 4 solution, 1.77 mol of an FeSO 4 solution and 37.0 mol of an aqueous NaOH solution were added to a dispersion at 90°C which was obtained by dispersing 1856 g of octahedral magnetite particles (average particle diameter: 0.25 µm, BET specific surface area: 6.3 m 2 /g Fe 2+ content: 20.2 wt%) produced from an aqueous solution (by wet-process) into water, while blowing N 2 gas into the dispersion at a rate of 20 liter/min, so that the total content was 41.0 liter. The Co content and the Fe 2+ content in the obtained dispersion were 3.5 mol% and 7.35 mol%, respectively, based on the total Fe content in the granular magnetite particles, and the concentration of the OH group was 0.90 mol/liter. Thereafter, air was blown into the resultant dispersion in place of the N 2 gas at a rate of 100 liter/min, while the resultant dispersion was stirred at a temperature of 90 °C for 20 minutes, thereby coating the granular magnetite particles with cobalt ferrite. The black precipitates obtained were filtered out, washed with water and dried at a temperature of 60 °C to obtain black particles. As a result of the measurement and the composition analysis of Co and Fe by the method which will be described in the following, the composition of the surfaces of the obtained black particles was Co x Fe 2+y O z , wherein x = 0.97 and 2 + y = 2.03, and the Co content in the coating material was 3.5 mol% based on the total Fe content in the black particles before treatment and the Fe content in the Co x Fe 2+y O z layer was 7.3 mol% based on the total Fe content in the black particles before treatment. The BET specific surface area of the obtained black particles was 5.9 m 2 /g and the Fe 2+ content was 20.5 wt%. As to the magnetic characteristics of the obtained black particles, the coercive force (Hc) was 198 Oe, the residual magnetization (σ r ) was 20.4 emu/g, and the saturation magnetization (σ s ) was 85.6 emu/g. The exothermic reaction initiating-temperature measured from differential thermal analysis was 250°C. In other words, the coated granular magnetite particles had excellent in heat resistance. The Fe 2+ content after heating the granular magnetite particles at a temperature of 200°C for 60 minutes was 19.7 wt%. Since the Fe 2+ content before heating was 20.5 wt%, the change ratio (%) in the Fe 2+ content was as small as 3.9 %. The granular magnetite particles assumed a bluish black color, and the tinting strength was excellent. ΔE was 0.44. For the composition analysis, 10 g of the obtained black particles were suspended in 100 ml of water, and the resultant suspension was heated to 60°C in a reaction vessel. Thereafter, 200 ml of 1-N HCl solution was added to the suspension under stirring so as to measure the Co content and the Fe content. Six test samples were prepared and were taken out one after another t = 0, 1, 5, 10, 30, 60, and 120 minutes after the addition of the HCl solution to analyze the Co content and the Fe content in the filtrate obtained by filtering out the black particles. Examples 12 to 20, Comparative Examples 5 to 8 The coated granular magnetite particles were obtained in the same manner as in Example 11 except for varying the kinds of granular magnetite particles, the kinds and the amounts of aqueous cobalt salt solution, aqueous ferrous salt solution and aqueous NaOH solution in the mixing process, and the temperature in the oxidation process. The synthesis conditions and the resultant properties of the coated granular magnetite particles obtained are shown in Tables 4 to 6. Comparative Examples 2 and 6 were carried out as Example 1 except that the mixing process was conducted in air without blowing N 2 gas. It was observed by electron microscopy that fine zinc ferrite particles (Comparative Example 2) and fine cobalt ferrite particles (Comparative Example 6) formed on the particles obtained together with octahedral magnetite particles. Similarly, in Comparative Examples 3 and 7, it was confirmed by electron microscopy that fine zinc ferrite particles (Comparative Example 3) and fine cobalt ferrite particles (Comparative Example 7) formed on the particles obtained together with octahedral magnetite particles. Example 21 1 kg of magnetite particles coated with zinc ferrite as obtained in Example 1 were dispersed in water at 80°C, and 14.8 g of water glass #3, which was 0.2 wt% (calculated as Si) based on the said zinc ferrite coated particles, was added and the resultant dispersion was stirred for 10 minutes. Sulfuric acid was added dropwise to the dispersion so as to adjust the pH to 5.5, and the resultant dispersion was stirred for 15 minutes. The black precipitates obtained were filtered out, washed with water and dried at a temperature of 60°C to obtain black particles. As a result of the following fluorescent X-ray analysis, it was found that the black particles contained 0.19 wt% of Si. The amount of charge measured by the following method was -21 µC/g. The fluorescent X-ray analysis was conducted in accordance with JIS-K-0119 "General rule for fluorescent X-ray analysis" by using Fluorescent X-ray Analyzer Model 3063M (manufactured by Rigaku Denki Kogyo Co., Ltd). The charge was measured by using a blow-off charge measuring apparatus (manufactured by Toshiba Chemical Co., Ltd) after 30-minutes' friction between the particles and an iron powder carrier of TEFV-200/300 (produced by Nihon Teppun Co., Ltd). Examples 22 to 23 The Si-coated magnetite particles were obtained as in Example 21 except for varying the kind and the amount of Si compound and the pH of the dispersion. The synthesis conditions and the properties of the Si-coated magnetite particles are shown in Table 7. Example 24 1 kg of magnetite particles coated with zinc ferrite as obtained in Example 9 were dispersed in water at 65°C, and an aqueous NaOH solution was added dropwise under stirring so as to adjust the pH to 11. Thereafter, an aqueous solution containing 12.7 g of A1 2 (SO 4 ) 3 , which was 0.2 wt% (calculated as Al) based on the said zinc ferrite coated particles, was added dropwise to the dispersion and the resultant dispersion was stirred for 10 minutes. Sulfuric acid was then added dropwise to the dispersion so as to adjust the pH to 7.5, and the resultant dispersion was stirred for 15 minutes. The black precipitates obtained were filtered out, washed with water and dried at 60°C to obtain black particles. The black particles contained 0.19 wt% of Al and the measured charge was -5 µC/g. Examples 25 and 26 The Al-coated magnetite particles were obtained as in Example 24 except for varying the kind and the amount of Al compound and the pH of the dispersion. The synthesis conditions and the resultant properties of the Al-coated magnetite particles are shown in Table 7. Example 27 1 kg of magnetite particles coated with cobalt ferrite as obtained in Example 16 were dispersed in water at 70°C, and an aqueous solution containing 18.4 g of TiOSO 4 -2H 2 O, which was 0.45 wt% (calculated as Ti) based on the said cobalt ferrite coated particles, and an aqueous NaOH solution were simultaneously added dropwise to the dispersion while maintaining the pH at 7.0. The resultant dispersion was stirred for 15 minutes. The black precipitates obtained were filtered out, washed with water and dried at a temperature of 60°C to obtain black particles. The black particles contained 0.42 wt% of Ti and the measured charge was -23 µC/g. Examples 28 and 29 Ti-coated magnetite particles were obtained as in Example 27 except for varying the kind and the amount of Ti compound and the pH of the dispersion. The synthesis conditions and the resultant properties of the Ti-coated magnetite particles are shown in Table 7. Example 30 1 kg of magnetite particles coated with cobalt ferrite as obtained in Example 19 were dispersed in water at 75°C, and an aqueous NaOH solution was added dropwise under stirring so as to adjust the pH to 11. Thereafter an aqueous solution containing 18.5 g of water glass #3, which was 0.25 wt% (calculated as Si) based on the said cobalt ferrite coated particles, and an aqueous solution containing 28.5 g of Al 2 (SO 4 ) 3 , which was 0.45 wt% (calculated as Al) based on the the said cobalt ferrite coated particles, were added dropwise and the resultant dispersion was stirred for 10 minutes. Sulfuric acid was then added dropwise so as to adjust the pH to 8.5, and the resultant dispersion was stirred for 15 minutes. The black precipitates obtained were filtered out, washed with water and dried at a temperature of 60°C to obtain black particles. The black particles contained 0.25 wt% of Si and 0.41 wt% of Al, and the measured charge was -14 µC/g. Examples 31 and 32 Si- and Al-coated magnetite particles were obtained as in Example 30 except for varying the kinds and the amounts of Si compound and Al compound and the pH of the dispersion. The synthesis conditions and the resultant properties of the Si and Al-coated magnetite particles are shown in Table 7.

Claims: Coated granular magnetite particles each comprising a granular magnetite core particle coated with a layer of M x Fe 2+y O z wherein M is Zn or Co, 0.4 ≤ x ≤ 1, x + y = 1, 4.0 ≤ z ≤ 4.3 and the amount of M in said M x Fe 2+y O z is from 0.5 to 6 mol% based on the Fe content of the core particle. Particles according to claim 1, wherein M is Zn and the amount of Zn in the Zn x Fe 2+y O z is from 0.5 to 4.0 mol% based on the Fe content of the core particle. Particles according to claim 1, wherein M is Co and the amount of Co in the Co x Fe 2+y O z is from 1.0 to 6.0 mol% based on the Fe content of the core particle. Particles according to any one of the preceding claims, wherein the Fe 2+ content in the core particle is from 14 to 24 wt%. Particles according to any one of the preceding claims, wherein the BET specific surface area of the core particle is from 3 to 15 m 2 /g. Particles according to any one of the preceding claims, wherein the coercive force is not more than 300 Oe, the change after heating at a temperature of 200°C for 1 hour of the Fe 2+ content is not more than 6.5% and of the hue is not more than 0.65. Particles according to any one of the preceding claims, comprising a further coating layer which is present on the M x Fe 2+y O z layer, and which comprises a hydroxide, a hydroxide oxide or a mixture thereof of at least one element selected from Si, Al and Ti in an amount of from 0.1 to 5 wt% (calculated as Si, Al and/or Ti) based on the coated granular magnetite particles. Particles according to any one of the preceding claims capable of being charged to from -50 to 15 µC/g. A process for producing coated granular magnetite particles as defined in any one of the preceding claims, which process comprises: (a) forming an aqueous dispersion of granular magnetite core particles; (b) adding to and mixing with the said dispersion in a non-oxidizing atmosphere from 1.0 to 39 mol% of an aqueous ferrous salt solution (calculated as Fe 2+ and based on the amount of Fe of the core particle), from 0.5 to 6 mol% of an aqueous M salt solution (calculated as M and based on the amount of Fe of the core particle) and an aqueous alkali hydroxide solution, thereby adjusting the concentration of OH - groups in said dispersion to from 0.3 to 1.0 mol/l; and (c) blowing a molecular oxygen-containing gas into said dispersion at a temperature of not lower than 50°C to coat the surfaces of said core particles with M x Fe 2+y O z wherein M is Zn or Co, 0.4 ≤ x ≤ 1, x + y = 1, and 4.0 ≤ z ≤ 4.3. A process according to claim 9, further comprising: (d) adding at least one compound selected from an Si compound, an Al compound and a Ti compound to the reaction system resulting from step (c); and (e) adjusting the pH of the resultant aqueous dispersion to from 5 to 9 to coat each of said M x Fe 2+y O z layers with a hydroxide, a hydroxide oxide or a mixture thereof of at least one element selected from the group consisting of Si, Al and Ti. Coating materials, printing inks and resins incorporating particles as defined in any one of claims 1 to 8 as a colouring pigment, and magnetic toners and magnetic carriers incorporating particles as defined in any one of claims 1 to 8.

Inventors: AOKI KOSO, FUJIOKA KAZUO, KOZAWA MINORU, KURITA EIICHI, MISAWA HIROMITSU, OKANA YOJI, AOKI, KOSO, FUJIOKA, KAZUO, KOZAWA, MINORU, KURITA, EIICHI, MISAWA, HIROMITSU, OKANA, YOJI
Applicants: TODA KOGYO CORP, TODA KOGYO CORP.
Application year: 1994
Main classification: C09C1
IPC codes: G11B5, C09D5, H01F1, G03G9, C09C1, C09C3